Optical reflectometry techniques have long been employed in process control applications in the semiconductor manufacturing industry due to their non-contact, non-destructive and generally high-throughput nature. The continuous push towards developing smaller devices comprised of thinner layers and the introduction of complicated new materials has challenged the sensitivity of such instrumentation. As a result, this has necessitated an effort to develop optical reflectometry equipment utilizing shorter vacuum ultraviolet (VUV) wavelengths (below 200 nm), where greater sensitivity to subtle changes in material properties may be realized.
Common tools used in optical metrology techniques are reflectometry and ellipsometry. Ellipsometry is generally regarded as consisting of a “richer” dataset, including a measurement of two quantities per wavelength/incident angle. On the other hand, reflectometers are more robust due to less complex hardware configurations, have faster measurements, and typically have a smaller footprint. Generally speaking, if both technologies are capable of solving a given metrology problem, the reflectometer is a more cost effective choice for a high volume production environment.
Contamination of optical surfaces like windows and mirrors is a serious impediment to the operation of optical instruments in the VUV range. Moisture and residual molecules, particularly hydrocarbon compounds, may deposit on such surfaces over time dramatically reducing their performance. These materials may form as a function of exposure to environment and/or out-gassing materials. Additionally, VUV and DUV radiation from the optical metrology tool itself may cause the formation of a contaminant film by reacting with accumulated airborne or out-gassed contaminants via a photo-deposition process. These effects have formed the focus of previous investigations owing to their impact on the design, development and performance of 193 and 157 nm lithographic exposure tools.
When present on the surfaces of samples under investigation, contaminate layers may significantly contribute to measured optical responses in the VUV yielding inaccurate and/or erroneous results. These effects are of particular concern when the samples are comprised of ultra thin films (<100 Å), whose thicknesses may themselves be comparable to the thicknesses of the contaminate layers. Airborne molecular contaminants (AMC) deposit on the surfaces of such samples significantly affecting reflectance in the VUV region. As used herein, AMC refers not only to typical contaminants deposited on samples, but also photo-deposited contaminants.
Reflectometer calibration in the VUV is difficult since reliable absolute reflectance standards do not exist and could not necessarily be maintained. In the past, new methods were developed to overcome these issues, such as for example in U.S. Pat. Nos. 7,282,703 and 7,511,265 and U.S. patent application Ser. Nos. 10/930,339, 11/789,686 and 12/072,878, the disclosures of which are all expressly incorporated by reference in their entirety. Some of these methods to calibrate reflectometers involved utilizing two calibration samples, measuring the intensity from both samples and analyzing the ratio of the two samples spectra to determine the properties of the samples and their absolute reflectances. In one embodiment, the calibration samples may be comprised of, but are not limited to, a bare silicon substrate with a native oxide layer and/or a 1000 Å SiO2 layer on a silicon substrate. This is only an exemplary composition of calibration samples. The techniques used previously work very well over a range of AMC buildup on calibration sites.
In the case where the thickness of deposited AMC exceeds some value, for example >40 Å, then the calibration sample measurements may be less accurate. This may be attributed to several reasons. For example, the AMC may be deposited over time in such a manner that an ever increasing fraction of the incident light becomes diffusely reflected. This could result in an error in the analysis of the measured spectrum. Or, the properties of the AMC film may not be uniform as a function of the film thickness. An inexact understanding of the optical properties of the AMC film, such as the case where the film consists of different components, could also lead to less accurate measurements.
In the operation of a VUV metrology instrument, previously disclosed methods provided techniques for minimizing, controlling and removing the deposited AMC layers that accumulated over time, such as for example in U.S. Pat. No. 7,342,235 and U.S. patent application Ser. Nos. 11/600,414 and 11/600,477, the disclosures of which are all expressly incorporated by reference in their entirety to overcome these issues. Though the previously described techniques provided an accurate means of calibration of a VUV optical metrology instrument, the measurements remained valid only over a specific range of deposited AMC thicknesses.
Calibration of the metrology equipment may also be dependant on the tool user. The equipment operator may have to continually check on the thickness of the AMC layers and determine if the layers were thick enough to compromise calibration accuracy. It would be beneficial to an automated semiconductor manufacturing environment if the responsibility of equipment calibration was shifted from the tool user to an automated system.